Fitzpatrick's extension will be for an additional six years, and is worth just under a maximum of $60 million, with $24 million guaranteed, according to a league source.
"I'm excited about it obviously," said Fitzpatrick, who called into NFL Network's "Around the League" on Friday.
"I'm not going to lie, during the season when this stuff goes on, you try to leave it out of your mind and not think about it. But it is one of those things where it is a relief to get done. I'm really excited for this year and the future with the Buffalo Bills."
Fitzpatrick would have been a free agent this offseason -- likely the top available quarterback -- but the former journeyman has been a great fit in Chan Gailey's offense to the point that the Bills did not draft a quarterback in the spring and have spent much of this season trying to get Fitzpatrick extended.
Fitzpatrick, 28, was informed of the development immediately after practice Friday.
The Associated Press contributed to this report.